Found this on Pinterest and decided to try it out for myself :
Next time you buy green onions from the store, don’t throw the root bulb portions away. Save the bottom bit of the onions and put them in a cup of water on the windowsill.
The onions will grow new plant stems from the root bulb. The post swears you can do this repeatedly, I have a feeling that the bulb will eventually use up its energy stores and your onions will become weak and thready over time… but we’ll see.
This is about a week’s growth from my first cutting. So it really does work. I’ll repeat the process after a second cutting and see if the plants do as well. I just change the water every couple days to keep it fresh and clean.
Try it and see! I might use more green onion if it was readily available like this.